Hi all,
We (and our mechanic) believe that our AP Roll Servo unfortunately has become faulty in our PA28 Archer III, equipped with a KAP100 AP. We noticed it when the AP suddenly decided to bank significantly while running in HDG mode. This has happened repeatably. During the investigation, our mechanic found burn marks in the plastic cover of the roll servo, hence our suspicion of it being the root cause. The same roll servo engine was repaired 7-8 years ago.
The problem is that our service org wants an astonishing 3900USD+VAT for a replacement and another 2000USD+VAT in core charge that is returned after our old servo is received by them and is accepted (on which terms I do not know at this moment).
Does anyone know of any other options that we might have? Unfortunately we don’t have the part number.
Thanks in advance everyone, any advice is much appreciated.
Kind regards,
Martin
martin-esmi wrote:
We (and our mechanic) believe that our AP Roll Servo unfortunately has become faulty in our PA28 Archer III, equipped with a KAP100 AP. We noticed it when the AP suddenly decided to bank significantly while running in HDG mode. This has happened repeatably. During the investigation, our mechanic found burn marks in the plastic cover of the roll servo, hence our suspicion of it being the root cause. The same roll servo engine was repaired 7-8 years ago
….
Does anyone know of any other options that we might have?.
You should connect the KAP100 to a computer via a serial port and run diagnostics. E.g. you can run the servo from the computer to verify that it works correctly (or not). You need a special cable, though. I got one which you can borrow.
(We’ve had two aircraft with KAP100 and being able to test it ourselves has saved time and money more than once.)
The KAP100 doesn’t have a PC interface -it requires the large ramp test set and breakout box.
https://www.bendixking.com/content/dam/bendixking/en/documents/downloads/006-08377-0001_1-kap-150-ki-525a-pilot-s-guide.pdf
The defect reported sounds more like a failure of the attitude indicator with loss of roll output.
wigglyamp wrote:
The KAP100 doesn’t have a PC interface -it requires the large ramp test set and breakout box.
You’re right – sorry. I was thinking of KAP140.
Hello Martin, send me a PM. På svenska går bra. :)
Yes; the trade tends to blame the servos and you end up paying 4 digits and then find it doesn’t fix it.
The servos are fairly easy to bench test but you need the MM to do it and I have MMs only for the KFC225 servos. If anyone has the MM for any other servos I would love to get them for my manuals collection (the login is available to any EuroGA participant on request) 
Lots of people have problems with the attitude indicator source.
wigglyamp wrote:
The defect reported sounds more like a failure of the attitude indicator with loss of roll output.
Peter wrote:
Lots of people have problems with the attitude indicator source.
Thank you for your responses, much appreciated! That is interesting, it would be financially nutty to pay all that money if it doesn’t solve the problem. I would say it’s nutty to pay that money even if it does solve the problem. Are these servo’s really this expensive?
Since it’s happening occasionally, meaning that the AP works for everything from 5 minutes to 30 minutes before the aircraft starts banking (usually to the left I believe), would we be able to identify that using servo checking equipment? Or are you saying that it’s very far fetched to believe it’s a servo issue and instead, likely a problem with the attitude indicator source?
Fly310 wrote:
Hello Martin, send me a PM. På svenska går bra. :)
Jag skickar PM, tack snälla :)
After 5-30 mins, unlikely to be the servo.
Which AP modes does it happen (or not happen) in?
King servos cost 2-3k to overhaul but the OH is quite limited; they don’t e.g. replace the motor or even the brushes (they told me that).
Peter wrote:
After 5-30 mins, unlikely to be the servo.
Which AP modes does it happen (or not happen) in?
I believe it happens in all modes, I’ve experienced it myself in HDG and NAV at least.
What is the most common error source with the attitude source indicator?
Try just flying in AP mode – wings level, pitch hold. Steer with the rudder pedals and manual trim as necessary.
That eliminates heading issues (fluxgate, HDG valid, etc) and barometric (ALT hold) etc issues.
